Data multiplexer with simultaneous multiple channel capability

ABSTRACT

Capability is provided to dynamically reallocate the multiplex data stream from one communication channel to a plurality of communication channels. This allows increased flexibility in responding to degradation of the main communication channel and in accommodating increased user demand exceeding the capacity of the main channel. A table is generated to control the DTE PORTs accessed during each slot of a multiplexer frame. A separate table is generated to control the allocation of multiplex data between communication channels. Both tables are generated based upon a common Hash table. A plurality of Hash tables correspond to different output data rates and permit the output rate of the multiplexer to coincide with the maximum output rate capacity of an associated transmission device such as a modem.

This is a continuation of application Ser. No. 07/531,829, filed Jun. 1,1990 and now abandoned.

FIELD OF THE INVENTION

This invention relates to data multiplexers and more specificallyrelates to a multiplexer capable of dynamically reallocating part of amultiplexed data stream from a main channel to an auxiliary channel tohandle the total data throughput demand. This invention is furtherdirected to a data multiplexer capable of dynamically changing its datarate based on the quality of the communication channel.

BACKGROUND OF THE INVENTION

A conventional multiplexer receives data from a plurality of datasources or data terminal equipment (DTE) and combines the data fortransmission over a single high speed communication channel. In order toaccommodate the data throughput demand, the communication channelcapacity must at least equal the aggregate data rate from the datasources.

Bit multiplexers operate on received data on a bit by bit basis. Thesemultiplexers require that the ratio of the aggregate data rate to thesignalling (baud) rate be an integer. Also, the ratio of the bit rate ofeach DTE to the baud rate must be an integer. It will be apparent thatthese conditions place restrictions on the bit rates and baud rateswhich can be used.

Statistical multiplexers require that the aggregate transmission rate beequal to the sum of a subset of DTE port rates. For example, if DTErates were 14.4, 2.4 and 2.4 Kbps, the statistical multiplexer wouldrequire the communication channel to be able to support at least 14.4Kbps. Thus, the highest DTE rate could not be accommodated by splittingthe throughput demand over two or more communication channels.

Multiplexing systems have used a feature known as restoral. Systems withthis feature provide a means for switching to an alternate transmissionchannel upon the failure of the main communication channel ordegradation below the maximum throughput rate required. If conditionsdictate switching to an alternate channel, the restoral feature switchesthe entire data stream to an alternate channel thereby abandoning allcommunications over the main channel.

Biplexers generate transmission data streams with equal data rates.Thus, they cannot take full advantage of transmission channels withdifferent throughput capabilities. Normally biplexers split a user datastream into multiple data streams which are transmitted over separatetransmission devices. Any change in the data rate or failure of achannel will result in a drop in the available user data rate.

OBJECT OF THE INVENTION

It is an object of the present invention to provide an improved datamultiplexer capable of diverting part of the data being transmitted overa primary communication channel to a second communication channel eitherin response to increased user data demand or upon the main communicationchannel degrading below the required data throughput rate.

It is a further object of the present invention to provide an improveddata multiplexer capable of selecting from among a plurality of baudcommunication rates dependent upon the quality of the communicationchannel.

These objects and other benefits of the present invention can beascertained by reference to the following description of an embodimentof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an embodiment of a datamultiplexer in accordance with the present invention incorporated in adata communications system.

FIG. 2 is a diagram of the multiplexer as shown in FIG. 1.

FIG. 3 is a flow diagram illustrating the multiplexer training sequence.

FIG. 4 is a flow diagram illustrating the generation of a transmit(receive) leased line/dial line frame table utilized by the embodimentof the present invention.

FIG. 5 consists of Tables A1-A4 each illustrating the number (N) ofread/write accesses for DTE port rates for four multiplexer data (baud)rates.

Hash FIGS. 6, 7, 8, and 9 each correspond to a different multiplexerdata (baud) rate and define an access sequence of 256. The four Tablesshown in FIG. 2 are derived based on the Hash Tables shown in thesefigures.

Hash FIG. 10 Example illustrates an example of a multiplexer inaccordance with the present invention accommodating four DTE ports.Tables 42 and 56 of FIG. 2 are defined for this example by the HashTable.

Hash FIG. 11 Example illustrates the use of a Hash Table to defineTables 44 and 58 of FIG. 2. The Hash Tables define 256 accesses permultiplexer.

DETAILED DESCRIPTION

FIG. 1 illustrates a digital communication system in which multiplexer10 receives and transmits data to a plurality of digital terminalequipment (DTE 1-DTE 8). A "null" DTE 12 represents an address locationfor multiplexer 10 from which user data will never be present. This nullDTE can be selected by multiplexer 10 during slots in the multiplexerframe when user data is not being accessed.

A transmit/receiver (TX/RX) device 14 is coupled by means of a channel16 to multiplexer 10. In the illustrative example, device 14 comprises amodem which is connected to a leased telephone line 18 to a remotemultiplexer system 20.

Multiplexer 10 receives data from DTE 1-DTE 8 which may each beoperating at various data rates. The aggregate data rate received bymultiplexer 10 on line 22 equals the sum of the DTE data rates. In the"normal" mode of operation, multiplexer 10 multiplexes or combines thereceived data and provides a single output data stream coupled to modem14 by channel 16. The modem combines digital data bits into groups ofbits which are transmitted as a QAM signal over leased line 18 to acomplementary receiving modem which comprises part of multiplexer system20. This remote modem would also be coupled to a substantially identicalmultiplexer which would demultiplex or segregate the data back intocorresponding output ports to complementary DTEs in system 20.

One of the important aspects of the present invention resides inmultiplexer 10 being able to selectively route part of the incomingreceived data from the DTE's over a second or alternate communicationpath. A TX/RX device 22 which may also comprise a modem is connected tomultiplexer 10 by channel 24. Modem 22 is connected to a complementarymodem in multiplexer system 20 by the public switched telephone network(PSTN) 26. In general, multiplexer system 20 may consist of an identicalsystem such as shown to the left of the dashed line of FIG. 1.

In the normal mode of operation, multiplexer 10 transmits the input dataas a single output data stream by channel 16 to modem 14 and over leasedline 18 to multiplexer system 20. This assumes of course that thethroughput data capability of leased line 18 is at least equal to theaggregate data rate of DTE 1-DTE 8. Of course, modem 14 must also becapable of at least the aggregate data rate.

Circumstances may develop by which the aggregate input data rate exceedsthe capacity of leased line 18. For example, the initial system maycomprise only DTE 1-DTE 4 being used and providing an aggregate datarate approximately equal to the maximum data rate which can be supportedby leased line 18. Assume that the user wishes to add another DTE 5 (ormore) thereby exceeding the leased line capacity. In a conventionalmultiplexer, the user would have to substitute DTE 5 for one of theother DTEs of equal or greater data rate. The multiplexer 10 accordingto the present invention can accommodate the increased data demand bycreating a second multiplexer data stream over channel 24 to prevent theincreased user demand from exceeding the throughput data capacity of thesystem.

An increased user demand beyond the capability of leased line 18 mayoccur for a relatively short time. As shown, such additional data ishandled by a temporary connection between modem 22 and its correspondingmodem in system 20 via PSTN 26. This provides an economical solutionwhere the increased user demand is intermittent and such increaseddemand period lasts for a relatively short time. A constant increaseduser demand may justify the use of an additional leased line instead ofPSTN 26.

Another example where multiplexer 10 can be used to advantage occurswhen leased line 18 is operating at near capacity relative to userdemand and the quality of the leased line degrades such as due toweather conditions. For example, if leased line 18 is specified for amaximum data rate of 19.2 Kbps, the line may degrade to support only 9.6Kbps or 14.4 Kbps due to an impaired line condition. Under suchconditions, a conventional multiplexer would require the user to reducethe required data demand to accommodate the reduced leased linethroughput by denying access to one or more of the DTEs. Alternatively,a conventional multiplexer with a restoral feature might divert theentire multiplex data stream to an alternate channel.

The multiplexer according to the present invention provides an improvedsolution in the case of an impairment of leased line 18. The impairmentcan be sensed by device 14 and a signal conveyed by channel 16 tomultiplexer 10 indicative of the impairment. The modem may sense theimpairment either by an increased bit error rate or by a test whichmeasures the quality of the leased line. Multiplexer 10 responds to theindicated impairment by reinitializing itself and its complementarymultiplexer in system 20 so that the data rate over leased line 18 isreduced to the level of the degraded line capacity. An additional datastream over the PSTN is established to carry the remaining data. Apatent application entitled "Automatic Data Restoral for Modems", Ser.No. 460,780, and assigned to the assignee of the present invention, isincorporated herein by reference.

FIG. 2 illustrates a multiplexer 10 in accordance with the presentinvention. Switch S1 selects connections to data being transmitted fromDTE 1-DTE 8 and null 12 by terminals 28. Switch S3 selects connectionsto terminals 30 by which data is transmitted from multiplexer 10 to DTE1-DTE 8 and null location 12. Data from the DTE's coupled from switch S1is transmitted by path 32 to switch S2 which transmits the data toeither device 14 by path 16A or device 22 by path 24A or to null device15 by path 17. As used herein, LL and DL are abbreviations for leasedline and dial line, respectively. The switches S1 and S2 are controlledby the multiplexer (MUX) control 34 as indicated by control lines 36 and38, respectively.

The schematic representation of multiplexer 10 as shown in FIG. 2 hasbeen simplified in order to facilitate an understanding of theimprovements according to the present invention. Accordingly, it will beunderstood that MUX control 34 would typically consist of amicroprocessor system. Although data selection from among the DTE's isshown by means of a mechanical switch, it will be understood that thisfunction is normally implemented in electronic gates and registers ormemory which receive data from the DTEs. Since conventional multiplexersare known which use a microprocessor system, only those features whichrelate to the improvements of the present invention are described indetail.

DTE information is provided to MUX control 34 by inputs 40. Thisinformation includes the number of DTEs to be supported, the respectivedata rate of each, and the aggregate data rate for all DTEs. Thisinformation may be provided by means of manual switches located on thepanel of the multiplexer or may be transmitted to the multiplexer. Paths16C and 24C provide communication paths between MUX control 34 anddevices 14 and 22. Each device provides the MUX control 34 withinformation required such as the maximum data rate the respectivechannels are capable of running and the baud rate utilized by eachmodem. These lines also couple signals indicative of reduced linethroughput capacity resulting from line degradation from the respectivemodems to MUX control 34.

The transmit frame cycle table 42 and transmit LL/DL frame table 44preferably comprise lookup tables generated and utilized by themicroprocessor system of MUX control 34. The transmit frame cycle tabledefines a transmit frame and the number of times each of the DTE inputterminals 28 will be read by means of switch S1 and the time slots inwhich such connections are made. The TX LL/DL frame table determines foreach frame slot whether switch S2 will couple the DTE to modem 14 or 22or to null device 15.

The above descriptions with regard to FIG. 2 describe the multiplexer 10transmission function; the multiplexer receiver function is very similarand could be said to be the mirror image of the transmit function. Thereceived data stream or streams by modem 14 and 22 are converted todigital data and transmitted by channels 16B and 24B to first in-firstout delay elements 46 and 48, respectively. These represent controllabledelay elements having a delay controlled by multiplexer control 34.Where data is transmitted over the two separate channels, it will beapparent that different delays will likely be encountered. Thus, inorder to equalize the arrival of the received data, different delays canbe utilized. During the training of the modems 14 and 22 with theirrespective counterparts, the delay associated with each path can bemeasured and provided by lines 16C and 24C to MUX control 34. Thechannel with the shortest delay, typically the leased line, would haveits FIFO delay set to the differential delay time between the leasedline and the PSTN channel. The FIFO of the PSTN channel having thelonger delay is set to zero. These delay times in combination withconventional received data buffering associated with conventionalmultiplexers should be sufficient to ensure that data underflow oroverflow of the buffers do not occur due to different channel delays.This also preserves list time synchronization.

Switches S3 and S4 are controlled by MUX control 34 as represented bycontrol lines 50 and 52, respectively. Switch S3 and switch S4 arecomplementary to switches S1 and S2, respectively. That is, to beproperly synchronized, switch S3 will be connected to the same DTE asswitch S1 when the corresponding data was transmitted. Similarly, switchS4 will be coupled to receive the data from modem 14 or 22 dependent onthe original transmission as determined by switch S2. Switch S4 couplesthe received data by path 54 to switch S3.

Receive frame cycle table 56 and Receive LL/DL frame table 58 will beidentical to the corresponding tables 42 and 44 in the transmittingmultiplexer 10 from which the data is received. Table 56 represents atable which is utilized to control the distribution of data for eachframe slot to the DTEs by switch S3. Table 58 consists of a table whichcontrols from which channel the data will be received for each frameslot by control of switch S4. It will be understood by those skilled inthe art that the normal receive function of a multiplexer must becomplementary to the transmit function of the multiplexer in order toproperly recover and distribute original data.

FIG. 3 is a flow diagram illustrating the multiplexer training routine.Entry into the routine begins at START 100 and is followed by a trainingevent sense decision 102. Training events include initial power-up, achange of modem configuration such as change of baud rate, leased linedegradation requiring reconfiguration of data paths, and a receivedrequest to retrain from a remote multiplexer. If such an event is notsensed, this routine ends by branch 104. Upon sensing such an event, theinitiating multiplexer (MUX 1) sends a marker F0 followed by rateinformation to the receiving multiplexer (MUX 2) as indicated byfunction 106. Then by function 108, MUX 1 and MUX 2 initialize tables42, 44, 56 and 58. Next, MUX 2 sends a marker F1 to MUX 1 aftercompleting table initialization as indicated by function 110. Onreceiving marker F1, MUX 1 allows user data to be transmitted asindicated by function 112. Finally, MUX 1 transmits a marker F2 to MUX 2preceding the first user data according to function 114. The routinethen terminates at END 116. Thus, the transmitting and receivingmultiplexers are initialized in accordance with the above procedureprior to the transmission or reception of user data.

The multiplexer of the present invention uses fixed cyclic frames havinga predetermined number of slots, such as 256. In the illustratedembodiment, the multiplexer is capable of obtaining data from up toeight DTE ports during a frame. Switches S1 and S3 are designed toaccept or deliver a fixed number, such as 4, bits per access of eachport regardless of the data rate of the port. The multiplexer determineshow many times per frame each port must be accessed in order toaccommodate the port rate. The multiplexer can read or write to eachport at a maximum of twice per baud and must process two entries of theframe per baud. The following formula illustrates the number of accessesfor a particular port required during a frame to achieve a desired rate:

    N=data rate×256/[baud rate *8]

where N is the number of accesses to a particular port required during aframe, data rate corresponds to the data rate in bits per second of theparticular port, and baud rate is the baud rate of the correspondingmodem.

FIG. 5 illustrates the number of entries or slots per frame for the fourdifferent baud rates indicated in tables A1-A4. The "n/a" indicates thatsuch data rates are not available since the number of accesses requiredto accommodate such a rate would exceed 256. Thus, it will be apparentthat for higher baud rates higher data rates can be accommodated. Itshould be noted that the baud rates indicated for table A2 and A3consist of rounded numbers with the actual baud rate indicated by theassociated fraction.

Hash FIGS. 6, 7, and 9 are provided for each of the four baud rates.Each Hash table of FIGS. 6-9 represents a predetermined sequencing foreach entry from 0 to 255 for each multiplexer frame. It will be notedthat each Hash table is divided into four line groupings, each of whichrepresent 2.4 Kbps. Thus, a data rate of 1.2 Kbps can be achieved byutilizing two lines of each four line grouping with the other two linesallocated to another 1.2 Kbps port. For all of the Hash tables exceptFIG. 6, it will be seen that certain positions are not used, i.e. thesepositions represent null reads or writes.

Hash FIG. 10 illustrates an example of the use of Hash FIG. 8 for a userconfiguration as follows: PORT 1=2.4 Kbps; PORT 2=16.8 Kbps; PORT 3=1.2Kbps; PORT 4=1.2 Kbps; PORTS 5-8=not used. For the selected baud rate of2954, this example illustrates that the full user data throughputcapacity of 21.6 Kbps has been utilized. The particular access slots ineach frame are indicated for each of the four PORTS as shown in FIG. 10.This table determines the control of switch S1 and the correspondingswitch S3 in the receiving multiplexer. As a counter countsincrementally from 0 through 255, it will be apparent that differentPORTS are accessed according to each slot definition. For example, PORT1 would be accessed on the third count and accessed for the last timeduring the frame at count 249. TX frame cycle table 42 (and thecorresponding RX frame cycle table 56 in the receiving multiplexer)would each consist of a series of 256 entries with each entry having acorresponding address which identifies the PORT to be addressed duringthat interval. This table is generated based upon a selected Hash tabledependent upon the baud rate of the associated modem and on user datarequirements defined for each DTE PORT.

FIG. 4 shows a flow diagram which describes how tables 44 and 58 aregenerated from a selected Hash table. This sequence begins at step 200in which the appropriate Hash table is selected based upon the baud rateof the associated modem. Parameters J, D, and L are defined by steps202, 204, and 206, respectively. Parameters D and L define the number offrame slots in which data is to be transmitted (received) by the dialline channel and leased line channel. The Hash table selected by step200 will be the same Hash table as selected for the generation of tables42 and 56, i.e. corresponding to the leased line baud rate.

Following the setting of these parameters, the function H(J) is definedas SLOT in step 208 and corresponds to index position J in the Hashtable. In step 210, the TX DL/LL frame (SLOT) is defined equal to DIAL.Thus each function H(J) referenced in the Hash table is defined as aslot position where data is to be sent to the dial line by control ofS2. The J parameter is incremented in step 212 and step 214 makes adecision as indicated. The YES path creates a return to step 208 settingup a repetitive loop until J equals or exceeds D. This condition will besatisfied, i.e. the NO path will be selected, when the number of slotsdefined by the Hash table have been selected to satisfy the number ofbits per second required by the dial rate.

Step 216, 218, 220, and 222 set up a loop which functions similarly tothe preceding loop except that the TX DL/LL frame (SLOT) is defined asequal to LEASE. Thus, this loop defines a further series of slotsaccording to the selected Hash table sufficient to satisfy the bits persecond required according to the lease line data rate. These slots areselected consecutively according to the Hash table immediately followingthe selection of the dial rate slots. Step 222 will be satisfied whenconsecutive slots within the Hash table have been assigned to satisfyboth the dial rate and leased rate, as evidenced by exiting via the NOpath.

Steps 224, 226, 228, and 230 form the final loop in which any remainingslots in the Hash table not previously assigned as DIAL or LEASEaccording to the two preceding loops will be assigned equal to NULL.This function corresponds to time slots of no user data transmission.Thus, this loop effectively assigns any remaining slots either availablefor user data or assigned as never-used slot positions according to theHash table as a NULL. This loop ends when step 230 determines that allremaining slots through 255 have been assigned. This loop exits by theNO path.

The following example illustrates the generation of the TX (RX) LL/DLframe. Assume that the transmission baud rate is 2954 for a leased line;and an aggregate analog data rate of 21.6 Kbps is distributed as a dialrate of 9.6 Kbps and lease rate of 12 Kbps. FIG. 11 illustrates thisexample of the generation of the TX (RX) LL/DL table 44 (58) inaccordance with FIG. 4 and the Hash table. According to this example, Dequals 104 and L equals 130 indicating that the dial rate will require104 slots and the leased line rate will require 130 slots. As theselection proceeds according to the flow diagram in FIG. 4, the firstloop ending at step 214 will assign the first 104 slots in the Hashtable as DIAL. The second loop ending at 222 will assign the next 130slots in the Hash table as LEASE. The final loop ending at step 230 willassign the remainder of the 22 slots in the Hash table as NULL. Thus,all 256 frame slots have been assigned. It will be apparent that allslots within the frame available for user data has been utilized.

A further example with regard to FIG. 11 will illustrate thecorresponding assignment of PORT data by use of the same Hash table. Forthis example consider that the same requirements for the TX LL/DL tableas previously explained apply. Further, user defined requirements are asfollows:

PORT 1=2.4 Kbps;

PORT 2=16.8 Kbps; PORT 3=1.2 Kbps; PORT 4=1.2 Kbps; PORT 5-8=no use.Accordingly, the TX frame cycle table 42 will be generated by a routinesimilar to that explained with regard to FIG. 4 in which PORT 1 isassigned the first 26 entries corresponding to 2.4 Kbps, PORT 2 isassigned the next 182 (7×2.4 Kbps), PORT 3 the next 13 slots and PORT 4the final 13 slots to occupy all 234 available user slots. For thisexample, it will be apparent that all of the data for PORT 1 will besent over the DIAL line, the data for PORT 2 will be split andtransmitted over both the DIAL and LEASED lines, and the data from PORTS3 and 4 will be transmitted over the LEASED line.

From the above description, it should be noted that the multiplexeraccording to the present invention has a predetermined number ofthroughput data rates which correspond to different baud rates oftransmission by the modems. Although four such predetermined rates aredefined by means of Hash tables, it will be understood that differentnumbers of data rates can be accommodated by providing appropriatecorresponding Hash tables. Economies in terms of memory saving arerealized by using the same Hash table to generate both the TX (RX) framecycle tables which control the order of DTE access per frame slot andthe TX (RX) LL/DL frame tables which control the transmission channelutilized per frame slot. Although two communication channels were shownin the exemplary embodiment of the present invention, it will beapparent that more than two such channels could be utilized by applyingthe concepts of the present invention wherein switches S2 and S4 woulddistribute the data per slot in each frame appropriately among the datachannels.

The present invention provides greater flexibility than in conventionalmultiplexers and allows the ratio of the data rate for each PORT to thechannel signalling (baud) rate only to be a rational number. It mayconsist of an integer but it must only be a rational number. Multiplesignalling data rates are supported. Also the channel signalling ratesmay be asymmetric thereby allowing the signalling rate best suited for aparticular channel to be utilized; different communication channels donot have to operate at the same data rate.

Although the described examples assume input data from multiple DTE's,the multiplexer can be utilized to accept a single high speed data inputwith the capability of sending the corresponding data over two or morechannels. Also data transmission devices such as DSUs could be usedinstead of modems.

The present invention provides the improved capability of maintainingthe aggregate PORT rates corresponding to the user demand under adegrading primary communication channel condition by dynamicallyreinitializing and adding data transmission capacity over additionalcommunication channels. This capacity to dynamically add additional datatransmission capacity also can be utilized to accommodate an increaseduser data capacity demand, i.e. additional DTE requirements, by routingadditional data onto auxiliary transmission channels.

Although an embodiment of the present invention has been described andillustrated in the drawings, the scope of the present invention isdefined by the following claims.

What is claimed is:
 1. A baud clock driven data multiplexercomprising:means for receiving data from at least a first data terminalequipment (DTE) port; means for data multiplexing said received data toform a first data stream wherein said first data stream is a sum of datastreams of at least the first DTE port and is carried by a firstcommunication channel to a data demultiplexer that demultiplexes saidfirst data stream back to its original constituent data;quality-responsive diverting means, responsive to a signal thatindicates that a quality of the first communication channel that carriesthe sum of the data streams of at least the first DTE port has becomereduced such that said channel carries less data than said sum of datastreams, for automatically diverting part, but not all, of the receiveddata to a second data stream carried by a second channel so that saidfirst and second data streams together support transmission of saidreceived data over said first and second communication channels, wheresaid diverting means establishes communications over said secondcommunication channel only upon said quality of the first communicationchannel becoming reduced such that said first communication channel isunable to carry all of said first data stream.
 2. The multiplexeraccording to claim 1 wherein said receiving means receives data from aplurality of data terminal equipment (DTE).
 3. The multiplexer accordingto claim 1 wherein only one DTE is actively sending data to thereceiving means.
 4. The multiplexer according to claim 3 wherein saidsingle DTE provides data at a data rate substantially at the maximumthroughput rate of said first channel.
 5. The multiplexer according toclaim 1 further comprising means for selecting the data rate of saidfirst data stream from among a plurality of predetermined data ratesbased upon the data capacity of said first communication channel.
 6. Abaud clock driven data multiplexer comprising:means for receiving dataat a first aggregate data rate from at least a first data terminal (DTE)port; means for data multiplexing said received data to form a firstdata stream wherein said first data stream is a sum of data streams ofat least the first DTE port and is carried by a first communicationchannel having a fixed maximum data carrying capacity;quality-responsive diverting means for automatically diverting part butnot all of said received data to a second data stream carried by asecond communication channel upon receiving a signal that indicates aquality of said first communication channel that carries the sum of thedata streams of at least the first DTE port has become reduced such thatthe channel is unable to carry the received data at its aggregate datarate such that said channel carries less data than said sum of datastreams, and means for causing the diverting means to cease divertingpart of said data to said second data stream upon said firstcommunication channel's regaining quality such that said channel is ableto carry all of said received data whereby all of said received data ismultiplexed into said first data stream, wherein said diverting meansestablishes communications over said second communication channel onlyupon said first communication channel being reduced in quality such thatsaid first communication channel is unable to carry all of said firstdata stream.
 7. The multiplexer according to claim 6 wherein saidreceiving means receives data from a plurality of DTE.
 8. Themultiplexer according to claim 7 wherein said increase in received dataresults from an increase in the number of DTE providing said data. 9.The multiplexer according to claim 1 further comprising means forcausing said diverting means to cease diverting part of said data tosaid second data stream upon said first channel becoming able to carryall of said received data whereby all of said received data ismultiplexed into said first data stream.
 10. The multiplexer accordingto claim 6 further comprising means for selecting the data rate of saidfirst data stream from among a plurality of predetermined data ratesbased upon said fixed maximum data carrying capacity of said firstcommunication channel.
 11. The multiplexer according to claim 9 whereinsaid causing means releases use of said second communication channelupon all received data being carried by said first communicationchannel.
 12. The multiplexer according to claim 6 wherein said causingmeans releases use of said second communication channel upon allreceived data being carried by said first communication channel.
 13. Abaud clock driven method of multiplexing digital data comprising thesteps of:receiving digital data from a plurality of DTE's; datamultiplexing said digital data to form a first data stream, wherein saiddata stream is a sum of data streams from the DTES, and is carried by afirst communication channel; automatically diverting part, but not all,of said received data to form a second data stream carried by a secondchannel upon said first channel becoming reduced in quality such thatsaid first communication channel is unable to carry all of said firstdata stream that includes the sum of data streams from the DTES, saidfirst and second data streams together supporting transmission of all ofsaid received data over said first and second communication channels,respectively, and ceasing the diversion of said part of said digitaldata to a second data stream upon said first channel's regaining qualitysuch that the first channel is able to carry all of said received datawhereby all of said received data is multiplexed into said first datastream, wherein said diverting step includes establishing communicationsover said second communication channel only upon said firstcommunication channel becoming reduced in quality such that said firstcommunication channel is unable to carry all of said first data stream.14. The multiplexer according to claim 13 wherein said ceasing stepincludes releasing the use of said second communication channel upon allof said received data being carried by said first communication channel.15. A baud clock driven method for multiplexing digital data comprisingthe steps of:(A) receiving said digital data at a first aggregate datarate from at least a first data terminal equipment (DTE) port; (B) datamultiplexing said received data to form a first data stream wherein saidfirst data stream is a sum of data streams of at least the first DTEport and is carried by a first communication channel having a fixedmaximum data carrying capacity; (C) automatically diverting part, butnot all, of said received data to a second data stream carried by asecond communication channel upon receiving a signal that indicates thata quality of said first channel has become reduced such that saidchannel is carrying less data than said sum of data streams, and (D)ceasing the diverting of said part of said digital data to a second datastream upon regaining the quality of the first channel, wherein saiddiverting step includes establishing communications over said secondcommunication channel only upon said first communication channelbecoming reduced in quality such that said first communication channelis unable to carry all of said first data stream.
 16. The methodaccording to claim 15 further comprising the step of increasing saidreceived data to form an increased first aggregate data rate byaccepting data from additional data sources.
 17. The multiplexeraccording to claim 15 wherein said ceasing step includes releasing theuse of said second communication channel upon all of said digital databeing carried by said first communication channel.
 18. A baud clockdriven data multiplexer comprising:means for receiving data from aplurality of DTE's, said data having an input aggregate data rate; meansfor data multiplexing said received data together to form a first datastream, wherein said first data stream is a sum of data streams of theDTEs, carried by a first communication channel having a maximum datarate capacity; means for automatically selecting a data rate for saidfirst data stream from among a predetermined number of rates such thatthe selected data rate is the highest data rate of said predeterminedrates that does not exceed said maximum data rate capacity of said firstcommunication channel, and quality-responsive diverting means forautomatically diverting part of said received data to a second datastream carried by a second channel if said selected data rate becomesreduced in quality such that the first communication channel thatcarries the sum of the data streams of the DTE ports is unable to carryall of said first data stream.
 19. The multiplexer according to claim 18wherein said selecting means automatically selects a data rate less thansaid selected highest data rate upon said maximum data rate capacity ofsaid first channel decreasing below said highest data rate.
 20. A baudclock driven method for multiplexing data comprising the stepsof:receiving data from a plurality of DTEs, said data having an inputaggregate data rate; data multiplexing said received data together toform a first data stream wherein said first data stream is a sum of datastreams of the DTEs and is carried by a first communication channelhaving a maximum data rate capacity; automatically selecting a data ratefor the highest data rate of a predetermined rate that does not exceedsaid maximum data rate capacity of said first communication channel, andautomatically diverting part of said received data to a second datastream carried by a second communication channel upon the firstcommunication channel becoming reduced in quality such that said firstcommunication channel is unable to carry all of the sum of the DTE datastreams of said first data stream.
 21. The method according to claim 20further comprising the step of selecting a data rate less than saidselected highest data rate upon said maximum data rate capacity of saidfirst channel decreasing below said highest data rate.
 22. The methodaccording to claim 20 wherein part, but not all, of said received datais diverted to said second data stream with the remainder of saidreceived data forming part of said first data stream carried by saidfirst communication channel.